The present invention relates to an image forming apparatus comprising a liquid crystal shutter array which is driven by thin film transistors and which can ensure a sharp contrast even with an increased number of shutter apertures and with an increased number of time-division.
Various attempts have been made in order to apply a liquid crystal shutter array having an array of a plurality of shutter apertures composed of a liquid crystal to an electrophotographic printer head or the like. When a liquid crystal shutter array is used as an electrophotographic printer head, a plurality of fine apertures are arranged in one or more rows and a light beam emitted from an exposure light source is projected onto the rear side of the liquid crystal shutter array. Each shutter is opened (to permit the transmission of light) or closed (to prevent the transmission of light) by turning on or off a voltage applied across a liquid crystal, and extremely small spots of light transmitted through the shutters are combined to form an image or pattern.
In the conventional liquid crystal shutter arrays, a number (n) of scanning electrodes and a number (m) of signal electrodes are arranged in a matrix array and a large number of picture elements are constituted by liquid crystals which are capacitor-type load elements. In order to drive a liquid crystal shutter array of the type described, address signals are selectively, sequentially and periodically applied to respective scanning electrodes and, in synchronism with the address signals, predetermined information signals are selectively applied to the signal electrodes in parallel. According to such a time-division or time-sharing driving system as described above, as the number of time-division is increased, the ratio V.sub.ON (ON signal)/V.sub.OFF (OFF signal) approaches 1 (unity) as shown by the below equation so that the opening-closing efficiency of a liquid crystal element constituting a picture element is degraded. As a result, especially in the case of a liquid crystal shutter array, an optical signal with a sufficient S/N (signal-to-noise) ratio can not be applied. Therefore, there has been a defect that when such a conventional liquid crystal shutter array is used as an image exposure unit (printer head) of an electrophotographic printer, high-quality images cannot be obtained. ##EQU1## where 1/N: the duty ratio
1/a: the bias ratio; and PA0 V.sub.0 : the voltage applied.
In a liquid crystal shutter array in which such shutters as described above are arranged in an array consisting of a plurality of rows and a plurality of columns, these shutters are sequentially driven by a time-division driving scheme, but when the number of time-division is increased, the contrast at each shutter is degraded so that only a low-quality image is obtained. This is considered to be because an effective voltage applied across a liquid crystal layer drops with increase in number of time-division.